Until recently, no human epithelial cell model of any kind has been used successfully in studying the neoplastic transformation process. We have recently demonstrated for the first time that immortalized human bronchial epithelial cells can be malignantly transformed by a single 30 cGy dose of high LET alpha particles (Hei et al,, 1994). Transformed cells progress through a series of sequential steps, including altered growth patterns, anchorage independent growth, before becoming tumorigenic and metastatic to produce progressively growing tumors in the livers and lungs of affected animals. With the availability of the single particle beam at the Radiological Research Accelerator Facility, this currently-funded project can be expanded to examine the effects of single particle traversal on intercellular communication, and ultimately, on the promoting phase of malignant transformation in these immortalized human epithelial cells.